Poster of the Northern Peru Earthquake of 26 September 2005 - Magnitude 7.5

Tectonic Summary

This major earthquake occurred within the lithosphere of the oceanic
Nazca plate. The earthquakes of northern Peru and most of western
South America are due to strains generated by ongoing subduction of
the Nazca plate beneath the South America plate. The Nazca plate
moves east relative to the South America plate at a rate of about 7 cm
per year. It is overridden by the South America plate at the Peru-Chile
trench, west of the Peruvian coast, and sinks into the Earth's mantle
beneath the South America plate. The subducted Nazca plate is seismically
active to depths of about 650 km. This earthquake occurred in
a segment of the subducted plate that has produced frequent earthquakes
with focal depths of 100 km to 150 km beneath the Earth's
surface. A magnitude 7.2 earthquake in 1997 was situated 150 km to
the north of the 26 September 2005 earthquake, at a similar depth.

Earthquakes that have focal-depths between 70 and 300 km are
commonly termed 'intermediate-depth' earthquakes, as distinguished
from 'shallow-focus' earthquakes, having depths less than 70 km,
and 'deep-focus' earthquakes, having depths greater than 300 km.
Intermediate-depth and deep-focus earthquakes represent deformation
within subducted plates, rather than deformation at plate boundaries.
Intermediate-depth and deep-focus earthquakes typically
cause less damage on the ground surface above their foci than is the
case with similar magnitude shallow-focus earthquakes, but large
intermediate-depth and deep-focus earthquakes may be felt at
great distances from their epicenters.

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